How to check which process is using the external harddisk / pendrive / thumb drive

sudo fuser -m /mnt 

(it will list out the processid)

sudo kill -9 processid

Best image viewer - https://nomacs.org/

https://nomacs.org/ 

How to mount and umount the device in ubuntu?

to mount the device:

sudo lsblk

sudo mkdir /mnt

sudo mount /dev/sdc1 /mnt

to umount the device

sudo umount /mnt

or 

sudo umount /dev/sdc1

sudo udisksctl power-off -b /dev/sdc1

How to setup NTP server in ubuntu20?

Reference: https://www.digitalocean.com/community/tutorials/how-to-set-up-time-synchronization-on-ubuntu-20-04 

First, run apt update to refresh your local package index:

sudo apt update

Then, run apt install ntp to install this package:

sudo apt install ntp

ntpd will begin automatically after your installation completes. You can verify that everything is working correctly by querying ntpd for status information:

ntpq -p

If you want to setup a local server:

sudo vim /etc/ntp.conf
---
# Point to our network's master time server
server 192.168.1.234 iburst
restrict default
driftfile /var/lib/ntp/ntp.drift
minpoll 4
maxpoll 5

Then, you can create a drift file as following:

sudo vim /var/lib/ntp/ntp.drift
---
0.000

Finally you can restart the service:

sudo service ntp restart
sudo service ntp status

How to install wifi driver in ubuntu?

ref: https://github.com/McMCCRU/rtl8188gu

sudo apt-get install build-essential git dkms
git clone https://github.com/McMCCRU/rtl8188gu.git
cd rtl8188gu
make
sudo make install
cd .. 
rm -rf rtl8188gu
sudo reboot

How to restrict or limit a new ubuntu user from accessing other folder?

1. add a new user

sudo adduser newuser

2. add user to sudo group

sudo usermod -aG sudo newuser

3. add user to ssh group

sudo vim /etc/ssh/sshd_config

>>AllowUsers newuser

4. add restriction to the new user (space sensitive)

sudo visudo -f /etc/sudoers.d/newuser

>>newuser ALL=(ALL) ALL, !sudoedit, !/usr/bin/su, !/bin/su, !/bin/bash, !/bin/sh, !/usr/bin/chmod, !/usr/bin/chown, !/usr/bin/docker, !/usr/bin/passwd, !/usr/sbin/visudo

if we want to allow some command to be executed without key in password:

newuser ALL=(ALL) NOPASSWD: /usr/bin/find

5. check the new restriction

sudo visudo -c

If it is ok, you will the following message print out:

/etc/sudoers: parsed OK

/etc/sudoers.d/README: parsed OK

/etc/sudoers.d/newuser: parsed OK

6. apply folder restriction to current user folder

sudo chmod -R 700 /home/currentuser

7. reboot the machine and check the restriction

sudo reboot

cd /home/currentuser

sudo chmod -R 777 /home/currentuser

sudo docker ps

How to ssh or scp without password?

 

Method 1: Use SSH Key-Based Authentication

The most secure and recommended way is to set up SSH key-based authentication, which doesn't require you to include a password in the command.

1. Generate SSH Key Pair (if you don’t have one already):

    ssh-keygen -t rsa -b 4096

Save the key in the default location (~/.ssh/id_rsa).

2. Copy Public Key to the Remote Machine:

ssh-copy-id username@remote_host -p 22

Replace username and remote_host with your remote machine's username and IP address or hostname.

3. Run rsync without Password or ssh into another machine without Password:

rsync -avz /path/to/source/ username@remote_host:/path/to/destination/

ssh username@remote_host -p 22

4. Change folder permission on destination folder.

sudo chmod -R 777 destination_folder